This invention pertains to methods and apparatus for constructing non-vertically oriented concrete structures. By “non-vertically oriented” I mean that the structure is generally oriented at an angle of between zero degrees and sixty degrees from horizontal, although there is no precise upper limit on the angle with respect to the horizontal except that it is generally less than ninety degrees from horizontal.
Non-limiting examples of non-vertical structures include bridge spans, large beams or transfer girders for applications such as building frames and offset bridges, conveyor galleries, and conduits, either buried or aboveground or elevated such as pipelines and other duct ways.
A universal theme in constructing non-vertical structures, and bridge spans in particular, is that as the bridge spans are constructed or assembled, they progressively take the final design shape of the bridge. There are many ways this construct-in-place or assemble-in-place theme is accomplished: (1) the bridge spans can be constructed in stages on false work beams and bents, as is the case with most cast-in-place post tensioned highway bridges (an example is the standard cast-in-place post-tensioned box girder bridge); (2) steel or precast beams or girders can be set between bents or piers, and then spanned with steel decking or a form soffet between these beams or girders, and a concrete deck is then cast that is composite with the beams or girders (this method is commonly referred to as “composite bridge construction”); (3) whole bridge sections are assembled into a large portion of a span or a whole span and are then transported to a job site and set on support piers or bents (an example of this method is construction of a steel trestle bridge across a river, the sections of which are put in place by barge cranes); (4) precast or cast-in-place sections are progressively cantilevered off of a pier support through bending rigidity and/or support links such as cables from a tower until a complete span is achieved at an abutment or by meeting a span that also may be cantilevered off of a distantly adjacent pier (examples are concrete box girder viaduct construction as well as cable-stayed bridges); (5) suspension bridge construction; and (6) floating bridge construction.
There are a number of shortcomings with the prior art. Firstly, as concerns the achievement of the universal theme of constructing and/or assembling a bridge in its final orientation, in virtually all examples of construction described above, the means of temporary support such as false-work or the support equipment such as crane barges inherently constricts or blocks the very avenue the bridge is being constructed to cross over for the majority of the duration of the construction project. For example, false-work constricts freeways for months during construction. Secondly, the labor pool involved in construction of bridges and the like inherently has to travel to the work rather than work coming to the worker (i.e., a finished bridge is not delivered to a worksite for installation, but is constructed at the installation site). Geographically the area of construction activities for non-vertical structures is much greater and more dispersed than for vertically oriented structures (such as a building, for example), which requires more access ways and equipment such as cranes, and more equipment moves. Further, there are a significant number of varied activities associated with the prior art approaches to constructing non-vertical structures, which require more and varied supervision and a broader set of learning curves for persons working on the construction job, all of which are expensive and time consuming.
A further reason that such non-vertical structures are typically built-in-place is that the shear mass of modular pieces of precast concrete, and the massive mechanical means required to get them to an assembly point on a bridge span, generally precludes the use of very large precast units. It also makes it necessary to repeat very time consuming and precise fit-up activities as well as to replicate expensive connection details quite frequently along the length of the span. Accordingly, most bridges include conventionally-formed cast-in-place concrete sections. The forming and casting process tends to be very labor intensive, involving a significant number of skilled laborers such as carpenters and ironworkers.